1. Field of the Invention
This invention relates to a method of improving the mechanical properties of a .gamma./.gamma.'-.alpha. eutectic nickel-base superalloy body containing a .gamma.-phase, a .gamma.'-phase, and an aligned molybdenum fiber .alpha.-phase which comprises the steps (a) heating the body to a temperature at which at least a portion of the .gamma.'-phase will transform to a .gamma.-phase, (b) maintaining the heated body at said temperature to allow transformation of at least a portion of said .gamma.'-phase to a .gamma.-phase, and (c) cooling the transformed body to a temperature at which at least a portion of the .gamma.-phase precipitates as a modified .gamma.'-phase.
As is traditional in the nickel-base superalloy field, the nomenclature .gamma. refers to a solid solution of a face centered cubic crystal structure and the nomenclature .gamma.' refers to an ordered LI.sub.2 phase based on Ni.sub.3 Al where other elements may substitute for some of the aluminum.
2. Description of the Prior Art
Thompson and Lemkey (Report M110544-2, United Aircraft Corporation, April 1973) and Ashbrook ("AGARD Conference Proceedings No. 156", AGARD/NATO, April 1974, pp. 93-115) report a .gamma.'-.alpha. nickel-base eutectic consisting of Ni.sub.3 Al, designated .gamma.', matrix and molybdenum fibers, designated .alpha.. Thompson and Lemkey report that the alloy exhibits a melting point of 1,306.degree. C., a fiber volume fraction of 26%, a density of 8.18 g/cm.sup.3, a room temperature tensile strength of 1,120 MN/m.sup.2, an elongation to failure of 21%, and a modulus of elasticity of 138 GN/m.sup.2. Ashbrook reports that the Thompson and Lemkey data are based on a eutectic .gamma.'-.alpha. containing, on a weight percent basis (w/o), 65 Ni - 8.0 Al - 27.0 Mo.
Unexpectedly, as reported in the above-referenced copending U.S. patent application Ser. No. 691,758 filed June 1, 1976, I found that Thompson and Lemkey's composition is a .gamma.-.alpha. eutectic and is not a .gamma.'-.alpha. eutectic. I have found that the Thompson and Lemkey alloy freezes as a .gamma.-.alpha. eutectic and on subsequent cooling undergoes a solid state phase transformation in the matrix whereby the .gamma. is converted to substantially 100% .gamma.'. This finding of fact is based on the examination of photographs of the microstructures of a series of alloy specimens annealed near the melting point of the Thompson and Lemkey alloy which were subsequently rapidly quenched to preserve the microstructure representing equilibrium at the annealing temperature. Further, as a result of the work carried out during the evaluation of the unheat-treated alloys claimed in Ser. No. 691,758, I found unexpectedly that optimum stress rupture life values are associated with directionally solidified alloys which contain, on a weight percent basis, variable amounts of aluminum, e.g. wherein the aluminum content is less than 7.0, preferably less than 6.5 and more preferably less than 6.3 w/o Al for alloys containing three chemical elements, i.e. Ni, Al and Mo. Further I found that the optimum range for improved stress rupture properties for unheat-treated three elements Ni, Al and Mo alloys corresponds with a 5.7 to 6.3 w/o aluminum range.
Unexpectedly, I have now found that substantial improvement in the mechanical properties, e.g. stress rupture life, of .gamma./.gamma.'-.alpha. eutectic nickel-base superalloys can be obtained by heating the alloy to a temperature at which at least a portion of the .gamma.'-phase transforms into .gamma.-phase with subsequent cooling of the resulting melt and solid solution whereby at least a portion of the resolutioned .gamma.-phase precipitates as a modified .gamma.'-phase, i.e., a .gamma.'-phase of different grain size and/or spacing than the grain size and spacing associated with the unheated alloy. Heat treatment of directionally solidified .gamma./.gamma.'-.alpha. eutectic nickel-base superalloys substantially improves the mechanical properties of any .gamma./.gamma.'-.alpha. alloy, including those of 3-element Ni-Al-Mo alloys having less than 7.0 w/o aluminum as well as the prior art 65 w/o Ni-8.0 w/o Al-27 w/o Mo - .gamma.-.alpha. eutectic of Thompson and Lemkey subject to the proviso that the resulting alloy has a .gamma./.gamma.'-.alpha. alloy morphology. For a 3-element Ni-Al-Mo alloy any aluminum content can be employed which permits the formation of a .gamma./.gamma.'-.alpha. eutectic, e.g. 2-12 w/o aluminum, 12-40 w/o molybdenum, the balance being nickel. Further, and additionally, heat treatment related improvement in the mechanical properties of .gamma./.gamma.'-.alpha. eutectic alloys also occurs when the basic Ni-Al-Mo alloy types are further modified by the addition thereto of other alloy elements, such as Cr, Co, Ta, Ti, B, W, Re, Nb, Zr, Hf, Y and the rare earth elements, B or carbon, including combinations thereof.